Platform

Well, the "ideal" grid size would be the size of the biggest buildings in that zone. As for RCI, it goes as follows:
R=6x6
C=8x8
I=7x7

In the case of Farms and Harbors:
F=4x4 is the minimum, although they're very customizable so you can scale up that number really easily
H=2x2 to 4x4, and higher. Same as farms, they're highly customizable

I am experimenting with this and was trying to fit RCI in 3 8x8 squares.
Here's the map I am using